The present invention relates to electronic communication systems, and more particularly to systems in which multiple signals are simultaneously transmitted at varying power levels.
In electronic communication systems, it is often necessary that groups of information signals be amplified and transmitted simultaneously. For example, a cellular radio base station transmitter typically transmits signals to many active receiving mobile stations within a single geographic cell. The signals typically appear at multiple predetermined frequencies in such multi-carrier signals. Similarly, a satellite communications transponder amplifies and transmits large number of information signals destined for various participating remote stations. Because such systems customarily employ a frequency division multiple access (FDMA) scheme, in which information signals are modulated on signal carriers occupying several frequency channels within an allocated frequency band, care must be taken to avoid inter-channel interference which may corrupt signal transmissions.
One possible source of such cross-channel interference is known as intermodulation distortion (IMD), which may result when two or more signals of different frequencies are mixed. For example, if two carriers of different frequencies are amplified using a non-linear amplifier, spurious outputs occur at the sum and difference of integer multiples of the original carrier frequencies.
As described in detail below, third order intermodulation products resulting from two relatively strong signals may disrupt transmission of a third relatively weak signal being transmitted on a carrier having a frequency equal to the frequency of the intermodulation product.
Various solutions have been proposed for improving linearity and reducing inter-channel effects in multi-carrier amplifiers. One such solution is the feed-forward amplifier circuit. In the feed-forward amplifier two loops are used to cancel distortion. In a first loop, a portion of the signals at the input to the amplifier are fed forward and, foltlowing suitable amplitude and phase adjustment, are subtracted from the amplifier output to generate an error signal. The error signal is proportional to distortion components of the output. The first loop that generates the error signal is known as the signal-cancellation loop. The error signal is then amplified, phase-adjusted and subtracted from the amplifier output to give a corrected signal output with reduced distortion effects. This portion of the circuit is known as the error-cancellation loop.
In one design, a “pilot tone” is introduced to the first loop of the feed-forward amplifier (i.e. signal cancellation loop). Then, the amplitude and phase adjustments in the error cancellation loops are performed by varying the amplitude and phase until a desired output is obtained. The adjustments made in this manner, however, are inherently narrowband and give optimal amplifier performance only at a certain frequency.
Other possible approaches to multi-channel linearization are described in U.S. Pat. No. 5,077,532, in which a microprocessor adjusts the feed-forward circuits, based on spectral analysis of the amplifier output signal; and in U.S. Pat. No. 5,455,537, in which a broadband pilot signal is used. An article by Parsons, et al., entitled “A Highly-Efficient Linear Amplifier for Satellite and Cellular Applications,” in IEEE Globecom, Vol. 1 (December 1995), pp. 203-207, suggests combining analog predistortion with feed-forward linearization. PCT patent publication WO98/12800 describes an RF power amplifier that combines feed-forward correction with adaptive digital predistortion. An RF signal output by the power amplifier is down-converted and sampled, and the average power of this signal is used as an input to a look-up table, so as to vary the values of complex gain applied to the input signal to the amplifier and to a feed-forward error signal.
Accordingly, it would be desirable to provide other techniques that reduce intermodulation distortion to, for example, compensate for non-linearities introduced by power amplifiers in multi-carrier environments.
Those having skill in the art would understand the desirability of having a spurious ratio circuit for use with feed-forward linear amplifiers. This type of spurious ratio circuit for use with feed-forward linear amplifiers would necessarily tend to more completely cancel in-band distortion, thus allowing the linearity of the amplifier to be improved.
As discussed above, many prior art systems depend on injecting a “pilot” signal of known amplitude at some point prior to the distortion-cancellation loop, and by various means measuring its level at the linearized output, and optimizing the loop controls to minimize this. These techniques have the disadvantage that they introduce another spurious signal, and that they optimize at the pilot frequency, which may not necessarily be located within the frequency range of the wanted signal.
Other pilot-less techniques with prior knowledge of the distortion frequencies have simply measured the amplitude of the distortion, and attempted to minimize it. This requires that the input signals do not vary in level, so that any change in the output distortion is due only to adaption of the cancellation loop controls.
Still other techniques correlate the wanted RF signal at the input and output of the linearized RF amplifier (LPA). The quadrature outputs from the correlator are used to control the loops. This method has serious problems with the interference of spurious signals, and maintaining accuracy over a wide dynamic range, since it requires monitoring over a wide bandwidth.